A case of strong convective gradient winds
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A case of strong convective gradient winds

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    At 1:00 a.m. CDT Tuesday morning (June 20, 1989), the National Weather Service at Sioux Falls received a bit of a surprise. Radio station KWYR in Winner called to report that 50 to 70 mph winds at both Winner and Colome had knocked down some trees and power lines. In fact, KWYR was without power when they called and a little nervous about what was going on. Both Alliance and Huron radars were showing scattered thunderstorms over north central and south central South Dakota. Maximum tops of 40,000 feet barely reached the tropopause, but no storm intensity greater than DVIP 2 was in this area, or had been for more than one hour prior. Several meteorological parameters suggested this was a strong convectively-enhanced gradient wind case, rather than typical severe thunderstorm gusts. Consequently, a high wind warning was issued for zones 15, 16 and 19 in south central South Dakota (see South Dakota zone map, Fig. 1) Shortly after 2:00 a.m., a report was received of 50 mph winds south of Murdo. A telephone call to Pierre indicated that winds had suddenly picked up there as well, and had just gusted to about 40 mph. Also, since wind gusts to 45 mph had been reported at Valentine about 30 minutes before the Winner and Colome reports, it was obvious that the strong winds were translating northward. As a result, the high wind warning was extended to include zone 10. Later reports from the Winner area indicated that strong winds gusting to around 60 mph continued until about 3:00 a.m. CDT, or for two hours. Strong winds at Pierre, frequently gusting to 40 to 60 mph, lasted for slightly more than two hours, from 2:00 a.m. until around 4:15 a.m.
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